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Expression of a Xenopus Distal-less homeobox gene involved in forebrain and cranio-facial development.

Abstract
Homeobox-containing genes are thought to perform essential functions in the process of pattern formation in vertebrates and invertebrates. They provide cells with positional information critical for normal embryonic development. Since most of the identified homeobox genes in Xenopus seem to provide positional information for the development of the trunk, we have concentrated on genes that may be specifically involved in the formation of the head region. Using a polymerase chain reaction strategy we have searched for Xenopus homeobox-containing genes that might provide positional cues for correct development of the brain. In this paper we report the identification and cloning of a novel gene that by homology appears to be a member of the Distal-less homeobox gene family. We show that its temporal expression patterns in the cement gland, neural crest derived visceral arches, retina and forebrain, while quite diverse, does suggest shared developmental features which may be required for correct craniofacial development and the regionalization of the Xenopus brain. Furthermore, expression of this gene at later stages is primarily restricted to the tadpoleforebrain suggesting that the Distal-less gene product continues to play a role after the initial brain patterning is complete.

Fig. 4. (A) Whole-mount in situ hybridization to a stage 32embryo (anterior view) showing the most intense area of XDLL-1 expression in the forebrain - the lateral diencephalic region (arrow) (B) In situ hybridization of XDLL-1 to a stage 33/34 embryo (anterior view), At this stage the entire ventralforebrain is uniformly labeled. (C) A lateral view of the embryo in (B) showing the boundaries of XDLL-1 hybridization within the brain. Upper arrow (e) points at the epiphysis which delineates the dorsal limit of the telencephalon. The lower arrow (d) points at the dorsal limit of XDLL-1 expression boundary in the telencephalon which is ventral to the epiphysis. O, otic vesicle, upper triangle points to the plane of section in (H), lower triangle points to the plane of section in (G). (D) In situ hybridization of XDLL-1 to stage 42brain. Lateral view. Forebrain is to the left. t, telencephalon; d, diencephalon; i, infundibulum. Apparent signal in the hindbrain area is due to pigment granules and not to hybridization. (E) Wholemount hybridization to stage 43brain. Arrows indicate the plane of section in (F). (F) Cross section of the telencephalic region of the brain in (E) Notice the sharp boundaries of labeling. (G) Coronal section of the embryo in (c) at the level of pharyngeal arches (lower triangle in (C)). 1, ectoderm; 2, neural crest; 3, mesoderm; 4, endoderm. (H) Coronal section of the embryo in (C) at the level of the eyes (upper triangle in (C)). d, diencephalon; 1, lens; o, otic vesicle; r, retina; t, telencephalon. Notice the presence of hybridization in the retina and the absence of it in the lens.

Fig. 6. (A) Transverse section of the telencephalic region in Fig. 5A. Plane of section is indicated by (a). (B) Transverse section of the telencephalic region of the brain in Fig. 5A. Level of the section indicated by letter (b). (C) Transverse section through the mesencephalon and dorsal diencephalon of the brain in Fig. 5A. Plane of section indicated by and letter (c). (D) Coronal section of the brain in 5A. Plane of section is indicated by a letter (d). Arrow points to the strongly labeled area olfactoria primitiva.